1. Field of the Invention
The present invention relates to an active matrix display apparatus and, more particularly, to a display apparatus using an active matrix light-emitting panel having a light-emitting device such as an organic electroluminescence device.
2. Description of the Related Art
In an organic electroluminescence device (hereinafter, referred to as an organic EL device), the light emission luminance can be controlled by a current which flows through the light-emitting device. A matrix type display is being widely developed using a light-emitting panel having a plurality of light-emitting devices arranged in a matrix configuration. There are a simple matrix light-emitting panel and an active matrix light-emitting panel as light-emitting panels having the organic EL devices. The organic EL devices are merely arranged in a matrix configuration in the simple matrix light-emitting panel. On the other hand, a driving device including a transistor is added to each of the organic EL devices arranged in a matrix configuration in the active matrix light-emitting panel. The active matrix light-emitting panel has advantages such that an electric power consumption is smaller and an amount of crosstalk between pixels is smaller as compared with those of the simple matrix light-emitting panels and the like and, particularly, is suitable as a display of a large screen or a high precision display.
FIG. 1 shows an example of a circuit configuration corresponding to one pixel 10 of a conventional active matrix light-emitting panel. The circuit configuration is disclosed in, for example, Japanese Patent Application Kokai No.8-241057.
In FIG. 1, a gate G of an FET (Field Effect Transistor) 11 (i.e., a transistor for selecting an address) is connected to an address scan electrode line (i.e., an address line) to which an address signal is supplied. A source S of the FET 11 is connected to a data electrode line (i.e., a data line) to which a data signal is supplied. A drain D of the FET 11 is connected to a gate G of an FET 12 (i.e., a transistor for driving) and connected to the ground via a capacitor 13. A source S of the FET 12 is connected to the ground and a drain D is connected to a cathode of an organic EL device 15 and connected to a power source via an anode of the organic EL device 15. The light emission control operation of the circuit mentioned above will be described below.
When an ON voltage is supplied to the gate G of the FET 11 in FIG. 1, a data voltage is supplied to the drain D. When the gate G of the FET 11 is at an OFF voltage, the FET 11 enters into a cut-off state and the drain D of the FET 11 enters into an open state. For a period of time during which the gate G of the FET 11 is at the ON voltage, therefore, the capacitor 13 is charged by a voltage of the source S and the voltage across the capacitor 13 is supplied to the gate G of the FET 12. A current based on the gate voltage and the source voltage flows from the drain D of the FET 12 to the source S via the organic EL device 15, thereby allowing the organic EL device 15 to emit light. When the gate G of the FET 11 is set to the OFF voltage, the FET 11 becomes open and the FET 12 holds the voltage at the gate G by the charges accumulated in the capacitor 13 and maintains the driving current until the next scan. The light emission of the organic EL device 15 is also maintained. The operation similar to that mentioned above can be executed even if the capacitor 13 is not provided, since a gate input capacitance exists between the gate G and source S of the FET 12.
The circuit corresponding to one pixel of the display panel for performing the light emission control by the active matrix driving is constructed as mentioned above and the light emission of the one pixel is maintained in the case where the organic EL device 15 of the pixel is driven.
FIG. 2 is a diagram showing a light emission characteristics curve of the organic EL device 15, namely, characteristics of a light emission luminance (L) and a current (I) against a voltage (V) applied to the EL device 15 are shown by using a temperature of the EL device 15 as a parameter. The light emission characteristics of the organic EL device 15 are similar to diode characteristics of an electronic device. When the applied voltage is smaller than a light emission threshold voltage Vth, the current I is extremely small. When the applied voltage becomes larger than the threshold voltage Vth, the current I suddenly increases. The current I and the luminance L are almost proportional. By applying the driving voltage exceeding the light emission threshold voltage Vth, the organic EL device 15 emits the light with luminance which is proportional to the current according to the applied driving voltage as mentioned above. With respect to a temperature dependency of the light emission characteristics of the EL device 15, when the temperature of the EL device 15 is high, the light emission threshold value Vth is small and a light emission efficiency (namely, gradient or slope of the light emission characteristics curve) is large. As the temperature decreases, the light emission characteristics deteriorate, the light emission threshold value Vth rises, and the light emission efficiency deteriorates.
Luminance gradations of each light-emitting device have been controlled by changing an amplitude of a luminance control signal to the gate G of the FET 12 in the active matrix light-emitting panel mentioned above, since the source-drain current of the FET 12 as a driving current of the organic EL device 15 changes in accordance with the applied voltage to the gate G of the FET 12. In general, a luminance control using a digital signal obtained by A/D converting an analog luminance control signal is performed.
In a display apparatus for amplitude modulating a luminance control signal to the driving devices as mentioned above to adjust the luminance, however, if the digital signal obtained by A/D converting the luminance control signal is used as a control signal to the gate G, the number of gradations which can be displayed also changes in accordance with a change in amplitude of the luminance control signal. Resolution of an A/D converter of the luminance control signal deteriorates and the number of gradations which can be displayed decreases in accordance with the decrease in the amplitude of the luminance control signal when decreasing the luminance of the light-emitting panel. Therefore, there arises such a problem that a multigradation display with a high precision is impossible.
The present invention is made in consideration of the above problem and it is an object of the invention to provide a display apparatus in which even in case of changing a display luminance of a light-emitting panel, the number of gradations which can be displayed is not limited and the luminance can be easily changed and a multigradation display with a high precision can be performed.
According to the present invention, there is provided an active matrix display apparatus comprising: a light-emitting panel including a plurality of light-emitting devices arranged in a matrix configuration and driving devices for selectively allowing each of the plurality of light-emitting devices to emit light; a control circuit for controlling the driving devices in accordance with an input video signal; a light adjustment signal generating circuit for generating a light adjustment signal to designate a display luminance of the light-emitting panel; and a variable power source for supplying electric power of a magnitude according to the light adjustment signal to the plurality of light-emitting devices.
According to another aspect of the present invention, the variable power source is a variable voltage source to generate a voltage which changes in accordance with a control signal.
According to another aspect of the present invention, the apparatus further comprises a temperature detector for detecting a temperature of an environment where the light-emitting panel is placed, and wherein the light adjustment signal generating circuit includes a correcting circuit for correcting the light adjustment signal in accordance with a detection signal from the temperature detector.
According to another aspect of the present invention, the apparatus further comprises a photometer for measuring
a brightness of an environment where the light-emitting panel is placed, and wherein the light adjustment signal generating circuit includes a correcting circuit for correcting the light adjustment signal in accordance with a detection signal from the photometer.
According to another aspect of the present invention, a light emission period of the plurality of light-emitting devices is determined on the basis of a subfield 2n gradation method.
According to the present invention, there is provided an active matrix display apparatus including a plurality of light-emitting devices arranged in a matrix configuration and driving devices for selectively allowing each of the plurality of light-emitting devices to emit light, comprising: a light adjustment signal generating circuit for generating a light adjustment signal to designate a display luminance of the display apparatus; and a voltage converting circuit for converting a predetermined voltage into a voltage of a magnitude according to the light adjustment signal and supplying the converted voltage to the plurality of light-emitting devices.
According to the present invention, there is provided an active matrix display apparatus including a plurality of light-emitting devices arranged in a matrix configuration and driving devices for selectively allowing each of the plurality of light-emitting devices to emit light, comprising: a voltage converting circuit for receiving a light adjustment signal to designate a display luminance of the display apparatus, converting a predetermined voltage into a voltage of a magnitude according to the light adjustment signal, and supplying the converted voltage to the plurality of light-emitting devices.
According to another aspect of the present invention, the plurality of light-emitting devices are organic electroluminescence devices.